The field of the invention relates generally to gas turbines, and more particularly to systems and methods for use in adapting the operation of gas turbines during power grid under-frequency events.
Large increases in electrical power demand placed upon an electrical power distribution grid (“power grid”) may reduce the electrical operational frequency of the power grid and may cause an “under-frequency” event. In conventional electrical power generation systems that use one or more heavy-duty industrial gas turbines to supply electrical power to the grid, each turbine supplying power to the grid is synchronized to the electrical frequency of the power grid. As the operational speed of a gas turbine decreases, with other parameters being equal, the power output of the gas turbine generally decreases correspondingly. Consequently, during an under-frequency event, at least one gas turbine coupled to the grid may output a lower power.
Grid code regulations require that power production equipment have the capability to maintain load during under-frequency events. Various regions around the world have different requirements that must be satisfied before power equipment to be coupled to the grid is considered compliant. Typically, gas turbine generators satisfy the code requirements by increasing firing temperature, such that power output is increased. Increases in firing temperature increase power output at a given pressure ratio, which works when the gas turbine does not approach any operating limits, such as a pressure ratio limit or an inlet guide vane (“IGV”) position limit. A firing temperature increase is typically achieved by increasing the fuel flow supplied to a combustor in the gas turbine. The increased fuel flow creates a higher pressure at a turbine inlet, which in turn induces a higher back pressure on a compressor upstream from the turbine. Eventually, increasing the fuel flow results in reaching an operating limit of the compressor, such as the compressor pressure ratio limit, beyond which the compressor experiences a surge.
Generally, because under-frequency events are typically abnormal, rare, and time-limited, gas turbine control limits (“schedules”) that represent operational boundaries of the gas turbine, are relaxed to enable regulation-imposed power outputs during under-frequency events to be achieved. Often, the schedules are prepared in view of the capabilities of a gas turbine when it is initially manufactured, and typically do not take into account unit specific hardware variation and/or degradation of gas turbine components over time. Causing a gas turbine to operate in excess of the boundaries may cause increased degradation, may raise the risk of undesired gas turbine events, and may shorten the useful life of the turbine assembly.